This invention relates to a method of recovering alkali values from trona ore. In particular, this invention relates to treatment of mine water.
Trona ore is a mineral that contains about 85-95% sodium sesquicarbonate (Na.sub.2 CO.sub.3. NaHCO.sub.3. 2H.sub.2 O). A vast deposit of mineral trona is in southwestern Wyoming near Green River. By conservative estimates, this deposit contains about 75 billion metric tons of trona ore.
The sodium sesquicarbonate found in trona ore dissolves in water to yield approximately 5 parts by weight sodium carbonate (Na.sub.2 CO.sub.3) and 4 parts sodium bicarbonate (NaHCO.sub.3). To recover these valuable alkali products, the trona ore must be processed to remove insoluble materials and other impurities.
One such valuable alkali produced from trona is caustic soda (a commercial grade of sodium hydroxide). Caustic soda is one of the largest alkali commodities in the United States. Caustic soda is used in pulp and paper, chemical manufacture and water treatment; and supplies sodium in the manufacture of sodium compounds. Caustic soda is also used in soap, detergent, petroleum refining, textile and aluminum applications.
Typically, 50% caustic soda is produced from trona ore in a process known as "lime-soda (chemical) process, " which consumes great quantities of water. In that process, lime is mixed with water to produce "milk of lime" or calcium hydroxide. Milk of lime and sodium carbonate liquor are introduced into caustic reactors to form 12% caustic in a reactor circuit. The by-product, calcium carbonate, is separated from the caustic solution by way of a decantation or thickener system. The calcium carbonate, or "caustic tailings, " are washed to remove residual caustic before disposal or recycled to other processes.
Water is a scarce and valuable resource in Wyoming. Water is required to carry sodium carbonate in the form of sodium carbonate liquor into the caustic soda production process. In addition, water is used to wash the caustic tailings before underground placement of the caustic tailings. One source of water is river water from the nearby Green River. However, using river water in the caustic soda process and as a tailings wash is expensive and uses a limited resource.
Another substantial source of water is "mine water. " Mine water includes run-offs from tailings deposited in the mined-out cavities and other aqueous streams from the mining process. Because large amounts of tailings from above-ground sodium production processes have been disposed in mines, large quantities of mine water could accumulate in the mines, and could prevent access to trona ore deposits. Thus, it is necessary to remove mine water from such mines.
Mine water contains dissolved trona that can be used to supply sodium carbonate and sodium bicarbonate in sodium production processes. However, mine water also contains impurities from tailings that prevent use of mine water in soda ash production (and similar) processes that can be sensitive to impurities. In addition, mine water can create odor problems. Thus, typically mine water has been collected in large evaporation ponds. Crystallization occurring in such ponds results in crystals with impurities too expensive to separate. Therefore, it has not been commercially feasible to recover sodium values from these crystals.
Therefore, there is a need for a process to remove impurities from mine water so that the treated mine water can be used as a production vehicle in sodium-based chemical processes.